So it's told to cache a lump number something, and you claim that number changes each time.

Since you're targeting vanilla Heretic, I'd like to mention that the TEXTURE1 lump is suspiciously small. The IWAD's TEXTURE1 lump is 3102 bytes, yours is merely 112 bytes. I guess it only contains your new or modified textures, leaving all the unmodified IWAD textures out. And vanilla does not like that.

SLADE 3 has the option to import the texture definition from the base resource archive precisely for this reason. You probably told it to create it empty, or you didn't set the base resource archive at all. Either way, you'll have to import the textures from the IWAD TEXTURE1 lump now.